Combined cycle electric power plant with coordinated steam load distribution control

ABSTRACT

A combined cycle electric power plant includes gas and steam turbines and a steam generator for recovering the heat in the exhaust gases exited from the gas turbine and for using the recovered heat to produce and supply steam to the steam turbine. The steam generator includes a superheater tube through which a fluid, e.g. water, is directed to be additionally heated into superheated steam by the exhaust gas turbine gases. An afterburner further heats the exhaust gas turbine gases passed to the superheater tube. The temperature of the gas turbine exhaust gases is sensed for varying the fuel flow to the afterburner by a fuel valve, whereby the temperatures of the gas turbine exhaust gases and therefore of the superheated steam, are controlled. Loading and unloading of the steam turbine is accomplished automatically in coordinated plant control as a function of steam throttle pressure.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to the following co-assigned and copendingapplications which are hereby incorporated by reference:

(1) S.N. 399,790 filed by L. F. Martz et al on Sept. 21, 1975.

(2) S.N. 495,723

(3) S.N. 495,765

Reference is also made to related applications referred to in the aboveapplications, which related applications are also incorporated byreference. Reference is also made to the concurrently filed andco-assigned patent applications Ser. Nos. 564,563, 564,562, 564,571, and564,572. These applications describe distinctly different features of acombined cycle electric power plant with digital coordinated control.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a combined cycle electric power plantand more particularly to improved coordinated steam load distributioncontrol in a combined cycle plant.

2. State of the Prior Art

In the design of modern electric power plants, it is a significantobject to achieve the greatest efficiency possible in the generation ofelectricity. To this end, steam generators are designed to extract heatefficiently from and to use the extracted heat to convert a fluid suchas water into superheated steam at a relatively high pressure. Further,such steam generators have been incorporated into combined cycleelectric generating plants including both gas and steam turbines whereinthe exhaust gases of the gas turbine are used to heat water into steamthen to be transferred to the steam turbine. Typically, steam generatorsinclude a water heating section or economizer tube, a high pressureevaporator tube and finally a superheater tube, whereby water isgradually heated while increasing levels of pressure are applied theretoto provide from the superheater tube, superheated steam to supply thesteam turbine. A condenser is associated with the steam turbine toreceive the spent steam therefrom and for converting it into watercondensate to be fed back to the steam generator.

In a combined cycle electric power plant, the steam turbine is combinedwith a gas turbine whereby the heated exhaust gases of the gas turbine,otherwise lost to the atmosphere, are used to heat the circulated fluidand to convert it into steam to drive the steam turbine. In this manner,a significant reduction in the fuel required to heat the steam isachieved and the heat contained in the gas turbine exhaust gases iseffectively utilized. Further, an afterburner associated with theexhaust exit of the gas turbine serves to additionally heat the gasturbine exhaust gases, whereby the heat required to generate sufficientsteam to meet load requirements is provided. In particular, underconditions of relatively high loads, wherein the heat of the gas turbineexhaust gases is insufficient to supply the steam requirements, theafterburner is turned on to further heat the gas turbine exhaust gases.

In combined cycle operation, there is a particular need to coordinatethe control of the separate turbines, as well as the control of theafterburners. It is desired that the steam turbine be operated in whatis called a "turbine following" mode whenever the plant is supplyingelectrical power to a load, such that the steam turbine follows the gasturbines, with each afterburner positively following a respective gasturbine. In this mode, the steam produced by the gases exhausted by theafterburners is used in total by the steam turbine. In distributing loadamong the operating turbines, and determining the load change rates forthe respective turbines when responding to changed plant demand, theremust be coordinated control so as to optimize efficiency and responsetime. Further, there is required a control system which canautomatically determine which turbines and/or afterburners are incondition for coordinated control, or have been selected for coordinatedcontrol, and proceed with any combination thereof in coordinated controlwhile other elements are simultaneously under a lower level of control.Desirably, the coordinated control flexibility is available throughstartup, synchronization, and the full range of plant loading.

In the loading of the steam turbine of a combined cycle plant, it isdesirable that the steam turbine be automatically loaded subject only tothe overall plant load demand, i.e., that the distributive share of theplant load carried by the steam turbine be automatically controlled asthe steam turbine follows the gas turbines. In such automatic loading,or unloading, of the steam turbine, the load change rate should besubject only to monitored turbine operating conditions, e.g., steamthrottle pressure.

Reference is made to the above-listed copending applications dealingwith combined cycle power plants, and particularly to the description ofthe prior art set forth therein under the heading "Background Of TheInvention". The referenced applications, and in particular Ser. No.399,790, describe the basic system concept of coordinated plantoperation, and in particular disclose a specific analog embodiment of acoordinated system. The advantages offered by coordinated control,namely (1) better response and therefore better availability; (2) betterefficiency and therefore lower cost of operation; and (3) more organizedand better response to contingencies and therefore better reliability,are better achieved by incorporation of overall digital control at alllevels of operation. With overall digital control, additional andimproved control functions and loops can be incorporated, and theability to switch such functions in and out of operation, i.e., systemflexibility, can be vastly increased.

Reference is further made to Westinghouse Descriptive Bulletin 23-830,dated September, 1972, and entitled "PACE Automation and ControlSystem", which bulletin is also incorporated herein by reference. Thedescriptive bulletin summarizes the PACE System Control, including theplant operating modes and control panel layouts.

SUMMARY OF THE INVENTION

It is the primary object of this invention to provide a coordinatedcontrol system for a combined cycle electric power plant, the planthaving suitably two gas turbines and a steam turbine, which controlsystem has a subsystem for controlling loading of the steam turbine,providing for automatic loading of such steam turbine without operatorinput. In furtherance of this object, the control system of thisinvention provides for a plurality of modes of control operation,including a completely automatic mode and an operator automatic mode,with automatic selection means for determining which mode of operationis to be followed for the steam turbine. At two levels of digitalautomatic control the steam turbine valves are opened or closed at arate which is a function of determined steam throttle pressure state andstored information characteristic of the steam turbine.

At the highest level automatic mode of control, steam turbine load iscoordinated with the load carried by the gas turbines, the coordinatedload distribution determination being carried out together with thedetermination of steam turbine load or unload rate. Steam turbineloading is accomplished independently of operator input, being turnedover completely to automatic digital control. When loading, the steamvalves try to open completely to take the steam turbine to maximumavailable load, the rate of opening being controlled as a function ofthrottle pressure. Once the steam valves are fully opened, actual steamturbine load is determined as a following function of the gas turbineoperation.

In the lower level mode of automatic operation, steam turbine load isnot coordinated with the gas turbines, but load and unload rates arelikewise determined by a stored program digital computer as a functionof sensed steam throttle pressure. In either automatic mode ofoperation, coordinated load and load rate programs are carried oncontinuously and in sequence, and the throttle pressure state isdetermined periodically and in a sequence so as to provide a maximallyrecent determination of proper steam load or unload rate. Analog andmanual control modes are also provided, with the digital automaticcontrol having means for tracking so as to provide bumpless transferbetween analog and digital control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing showing the major operating units of acombined cycle electric power generating plant.

FIG. 2 is a schematic drawing showing the control levels of the controlsystem of the combined cycle electric power generating apparatus of thisinvention.

FIG. 3 is a more detailed diagram of the control system of thisinvention.

FIG. 4 is another block diagram showing the major hardware components ofthe control system of this invention.

FIG. 5 is a block diagram showing the organization of the digitalportion of the control system of this invention.

FIG. 6 is a block diagram representing the hierarchy of the digitalportion of the control system of this invention.

FIG. 7 is a block diagram showing a conceptual functional organizationof the control system of FIG. 1, for load control operation.

FIG. 8 is a block diagram showing the bidding sequence of the digitalcoordinated control operation of the system of this invention.

FIG. 9 is a functional block diagram of the coordinated control system.

FIG. 10 is a flow chart of the boiler startup monitor portion of thecontrol system.

FIG. 11 is a flow chart of the select operating mode portion of thecoordinated control sequence.

FIG. 12 is a flow chart of the steam turbine load distribution portionof the coordinated control sequence.

FIG. 13A is a flow chart of a portion of the steam/bypass valve controlportion of the digital subsystem; FIG. 13B is a drawing representing theload curve utilized in the control portion of FIG. 13A; and FIG. 13C isa drawing of the unload curve utilized in the control portion of FIG.13A.

FIG. 14 is a functional diagram of the gas temperature setpointsupervision portion of the coordinated control sequence.

FIG. 15 is a flow chart of the gas temperature setpoint supervisionportion of the coordinated control sequence.

FIG. 16 is a flow chart of the gas turbine load distribution portion ofthe coordinated control sequence.

FIG. 17 is a functional diagram of the HRSG1/AB1 control portion of thecoordinated control sequence.

FIG. 18 is a flow chart of the HRSG1/AB1 control portion of thecoordinated control sequence.

FIG. 19 is a representation of a portion of the operator panel dealingwith coordinated control.

FIG. 20 is a flow chart of the select/reject logic program for goinginto or out of coordinated control.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A. General Plant Description

Referring to FIG. 1 of the drawings, there is shown a functional blockdiagram of a representative embodiment of a combined cycle electricpower generating plant constructed in accordance with the presentinvention. Reference numeral 10 is used to identify the combined cycleplant as a whole. As such, the plant 10 includes a first gas turbine 12(sometimes referred to as "gas turbine No. 1", or GT1) which drives afirst electric generator 13. Fuel is supplied to the gas turbine 12 byway of a fuel control valve or throttle valve 14. Air enters the gasturbine 12 by way of a variable inlet guide vane mechanism 15 whichcontrols the degree of opening of the turbine air intake and which isused to adjust air flow during the startup phase and to increase partload efficiency. The fuel supplied by the throttle valve 14 is burned inthe gas turbine 12 and the resulting high temperature exhaust gas ispassed through an afterburner 16 and a heat recovery steam generator 18and is thereafter exhausted into the atmosphere.

Heat recovery steam generator 18 (sometimes referred to as "heatrecovery steam generator No. 1", or HRSG1) includes therein various setsof boiler tubes which are heated to a relatively high temperature by thegas turbine exhaust gas passing through the steam generator 18.Afterburner 16 (AB1) includes a burner mechanism for further increasingthe temperature of the gas turbine exhaust gas before it enters thesteam generator 18. Fuel is supplied to the burner mechanism in theafterburner 16 by way of a fuel control valve or throttle valve 19. Theprimary heat source for the steam generator 18 is the gas turbine 12,the afterburner 16 being in the nature of a supplemental heat source forproviding supplemental heat when needed. In terms of typical fuel usage,approximately 80% of the fuel is used in the gas turbine 12 and 20% isused in the afterburner 16.

The combined cycle plant 10 further includes a second gas turbine 22(sometimes referred to as "gas turbine No. 2", or GT2) which drives asecond electric generator 23. Fuel is supplied to the gas turbine 22 byway of a fuel control valve or throttle valve 24. Air enters the gasturbine 22 by way of a variable inlet guide vane mechanism 25 which isused to adjust air flow during turbine startup and to increase part loadefficiency. The fuel supplied by throttle valve 24 is burned in the gasturbine 22 and the resulting high temperature exhaust gas is passedthrough an afterburner 26 and a heat recovery steam generator 28 and isthereafter exhausted into the atmosphere.

Heat recovery steam generator 28 (sometimes referred to as "heatrecovery steam generator No. 2", or HRSG2) includes various sets ofboiler tubes which are heated to a relatively high temperature by thegas turbine exhaust gas passing through the steam generator 28.Afterburner 26 (AB2) includes a burner mechanism for further increasingthe temperature of the gas turbine exhaust gas before it enters thesteam generator 28. Fuel is supplied to the burner mechanism in theafterburner 26 by way of a fuel control valve or throttle valve 29. Theprimary heat source for steam generator 28 is the gas turbine 22, theafterburner 26 being in the nature of a supplemental heat source forproviding supplemental heating when needed. In terms of typical totalfuel consumption, approximately 80% of the fuel is used in the gasturbine 22 and 20% is used in the afterburner 26.

A condensate pump 30 pumps water or condensate from a steam condenser 31to both of the steam generators 18 and 28, the condensate flowing to thefirst steam generator 18 by way of a condensate flow control valve 32and to the second steam generator 28 by way of a condensate flow controlvalve 33. Such condensate flows through the boiler tubes in each of thesteam generators 18 and 28 and is converted into superheated steam. Thesuperheated steam from both of the steam generators 18 and 28 issupplied by way of a common header or steam pipe 34 and a steam throttlevalve or control valve 35 to a steam turbine 36 for purposes of drivingsuch steam turbine 36. The steam from the first steam generator 18 flowsto the header 34 by way of a steam pipe 37, an isolation valve 38 and asteam pipe 39, while steam from the second steam generator 28 flows tothe header 34 by way of a steam pipe 40, an isolation valve 41 and asteam pipe 42.

The spent steam leaving steam turbine 36 is passed to the condenser 31wherein it is condensed or converted back into condensate. Suchcondensate is thereafter pumped back into the steam generators 18 and 28to make more steam. Steam turbine 36 drives a third electric generator44.

A steam bypass path is provided for use at appropriate times fordiverting desired amounts of steam around the steam turbine 36. Thissteam bypass path includes a steam turbine bypass valve 45 and adesuperheater 46, the output side of the latter being connected to thecondenser 31 by way of a pipe 47. A drain valve 48 is provided for thefirst steam generator 18, while a drain valve 49 is provided for thesecond steam generator 28.

The operation of the combined cycle electric power generator plant 10 iscontrolled by a control system 50, typical control signal lines 51 beingshown in a broken line manner. As will be seen, the control system 50offers a choice of four different control operating levels providingfour different degrees of automation. From highest to lowest in terms ofthe degree of automation, these control operating levels are: (1) plantcoordinated control; (2) operator automatic control; (3) operator analogcontrol; and (4) manual control. The control system 50 includes ananalog control system which is constructed to provide complete and safeoperation of the total plant 10 or any part thereof. The control system50 also includes a digital computer that provides a real-time digitalcontrol system that works in conjunction with the analog control systemat the higher two levels of control to coordinate and direct theoperation of the analog control system. Failure of the digital controlcomputer results in no loss of power generation because the analogcontrol system provides for complete operation of the plant 10.

When operating at the highest level of control, namely, at the plantcoordinated control level, the control system 50, among other things,automatically coordinates the settings of the fuel valves 14, 19, 24 and29, the inlet guide vanes 15 and 25 and the steam turbine throttle andbypass valves 35 and 45 to provide maximum plant efficiency under staticload conditions and optimum performance during dynamic or changing loadconditions.

The control system 50 also enables a coordinated automatic startup orshutdown of the plant 10 such that the plant 10 can be brought from ahot standby condition to a power generating condition or vice versa in aquick, efficient and completely automatic manner. For example, theentire plant 10 can be started and brought to full load from a hotstandby condition in approximately 60 minutes time by having the plantoperator simply dial in the desired load setting and push a master plantstart button.

As an indication of the flexibility and reliability of the powergenerating plant 10, it is noted that the plant 10 can be operated inany one of the following configurations: (1) using one steam turbine andtwo gas turbines; (2) using one steam turbine and one gas turbine; (3)using two gas turbines only; and (4) using one gas turbine only. Thesteam turbine 36 will, of course, not operate by itself, it beingnecessary to use at least one of the gas turbines 12 and 22 in order touse the steam turbine 36. In order to obtain the benefits of combinedcycle operation, it is, of course, necessary to use the steam turbine 36and at least one of the gas turbines 12 and 22. When one of the gasturbines, for example the gas turbine 12, is not being used or is shutdown for maintenance purposes, then its associated steam generator 18can be removed from the system by closing its condensate flow valve 32and its steam isolation valve 38. When, on the other hand, the steamturbine 36 is not being used or is shut down for maintenance purposes,the steam generated by the steam generators 18 and 28 can be bypassed tothe condenser 31 by way of steam bypass valve 45 and the desuperheater46. As an alternative, when the steam turbine 36 is not being used,either one or both of the steam generators 18 and 28 can be drained andvented by the appropriate setting of condensate valves 32 and 33, steamisolation valves 38 and 41 and drain valves 48 and 49. In other words,each of the steam generators 18 and 28 is constructed so that itsrespective gas turbine can be operated with the steam generator in a drycondition.

The combined cycle plant 10 affords a high degree of reliability in thatfailure of any one of the major apparatus components will not reducetotal plant power generation capacity by more than 50%. In this regardand by way of example only, a combined cycle plant 10 has been developedhaving a nominal maximum power generating capacity of 260 megawatts. Insuch plant, each of the gas turbines 12 and 22 is capable of producing amaximum of approximately 80 megawatts of electrical power under ISOconditions (59° Fahrenheit at sea level) and the steam turbine 36 iscapable of producing a maximum of approximately 100 megawatts ofelectrical power. Thus, loss of any one of the turbines 12, 22 and 36,for example, would not reduce total plant capacity by as much as 50%.

It is noted in passing that the functional block diagram of FIG. 1 hasbeen simplified in some respects relative to the actual plant apparatusto be described hereinafter, this simplification being made tofacilitate an initial overall understanding of the combined cycle plant10. A major simplification in FIG. 1 concerns the fuel valves 14, 19,24, and 29. In operation, provision is made for operating the gasturbines 12 and 22 and the afterburners 16 and 26 on either of twodifferent kinds of fuel, namely, either natural gas or distillate typefuel oil. As a consequence, each of the gas turbines 12 and 22 and eachof the afterburners 16 and 26 is actually provided with two fuelthrottle valves, one for natural gas and the other for fuel oil. Also,various other valves and devices employed in the actual fuel supplysystems have been omitted from FIG. 1 for the sake of simplicity. Othersimplifications employed in FIG. 1 are of a similar character.

B. Combined Cycle Control Levels

Referring now to FIG. 2, there are shown the four control levels of thecontrol system 50 of this invention. The top, or highest available levelof plant control, is coordinated control. Under plant coordinatedcontrol, a digital computer generally directs the plant operationthrough startup, synchronization and loading to produce the plant powerdemand. The extent of coordinated plant control is dependent on theexisting plant configuration, i.e., according to the availability ofapparatus for operation or for coordinated operation. For example, if agas turbine is shut down, it is excluded from coordination. Similarly,if the gas turbine has been excluded from coordinated control by theoperator, the computer will operate accordingly, maintaining the otherturbines and afterburners in coordinated control. Coordinated controlprovides for optimum plant performance in any selected operatingconfiguration, i.e., any combination of selected turbines andafterburners may be under coordinated control. In this mode the digitalcomputer can automatically bring all three turbines from hot start tosynchronization to full power in about one hour, achieving optimum heatrate and producing the greatest megawatt power output from a givenamount of fuel.

The next highest available level of plant operation is the automaticcontrol, or operator automatic mode of control. In operator automatic,various control functions are performed by the programmed digitalcomputer, providing for automatic startup and automatic loading of thegas and stream turbines under the direction of the operator on aturbine-by-turbine basis. Thus, at this level of operation, control ofeach of the turbines can be considered to be automatic, subject tooperator setpoints, but there is no coordination of the opertion of eachturbine, or the afterburners. Any part of the system may be selected bythe operator for operator automatic control by use of appropriateswitches in the operation control center. Under operator analog control,the analog control portion of system 50 provides for control of theplant or any part of the plant by the plant operator. In this mode, anyselected plant configuration can be started up and loaded. The system iscapable of accepting transfer to operator analog from the coordinated oroperator automatic mode at any time. The digital and analog controlstrack each other, providing for bumpless transfer. Manual controls areprovided for backup of the normally automatic control loops.

It is to be noted that control system 50 offers a choice of coordinatedcontrol, operator automatic control or operator analog controlregardless of the combination of elements selected for power generation.It is not necessary for all the generating units in the system tooperate in the same mode at the same time.

C. Plant Control System

The plant control system 50 is organized to operate the plant equipmentsafely through startup and loading with high reliability so that theplant is highly and quickly available to meet power demanded from it. Toachieve this purpose, the plant control system is preferably embodied indigital/analog hybrid form, and the digital/analog interface ispreferably disposed in a way that plant protection and plantavailability are enhanced.

Generally, the total plant power is controlled by controlling theoperating level of the turbines and the afterburners, but the steamturbine goes into a follow mode of operation once the steam bypassvalves are closed and the steam turbine inlet valves are fully opened.In the following mode, the steam turbine produces power at a leveldependent on the steam conditions generated by the heat inputs to thesteam generators.

As shown in FIG. 3, the control system 50 includes a digital controlcomputer 58G, a digital monitor computer 100C and various analogcontrols for operating the plant equipment in response to processsensors 101C while achieving the described objectives. In this instancean automatic startup control for the steam turbine 36 is largelyembodied in the monitor computer 100C. An operator panel 102C providesnumerous pushbutton switches and various displays which make it possiblefor the plant to be operated by a single person. The pushbutton switchesprovide for numerous operator control actions including plant andturbine mode selections and setpoint selections.

In the operator analog or manual mode of operation, the operator setsthe fuel level for the gas turbines 12 and 22 and the afterburners 16and 26 through gas turbine controls 104C and 106C during loading, but ananalog startup control incuded in each of the gas turbine controls 104Cand 106C automatically schedules fuel during gas turbine startups. Inaddition, sequencers 108C start and stop auxiliary equipment associatedwith the gas turbines during gas turbine startups. The turbine bypasssteam flow and the turbine inlet steam flow are controlled by operatorvalve positioning implemented by a steam turbine control 110C duringsteam turbine startup and loading in the operator analog mode. Certainautomatic control functions are peformed for the steam and gas turbinesby the controls 104C, 106C and 110C in the operator analog mode.

In the operator automatic mode, the computers 58G and 100C performvarious control functions which provide for automatic startup andautomatic loading of the gas and steam turbines under the direction ofthe operator on a turbine-by-turbine basis. Afterburner controls 112Cand 114C and boiler controls 116C and 118C operate under operatorsetpoint control during the operator analog and operator automaticmodes. Respective digital/analog hybrid circuits 120C, 122C and 124Cinterface the digital and analog controls.

Under plant coordinated control, the computer 58G performs all of thedigital control functions that can be assigned to it, directing theplant operation through startup, synchronization and loading to producethe plant power demand. In all coordinated control cases, the boilercontrols 116C and 118C react automatically to operator setpoints andsignals generated by the process sensors 101C to control the steamgenerators according to plant conditions produced by coordinated turbineand afterburner operations. The computer 58G provides setpoint signalsfor the afterburners in the coordinated control mode but not in theoperator automatic mode. The boiler controls further are supervised bythe afterburner setpoint signals which are received at the analogcontrol center. Coordinated control provides the highest available levelof plant automation, and the operator automatic and operator analogmodes provide progressively less automation. Some parts of the analogcontrols function in all of the plant modes.

Generator synchronization is performed by a synchronizer 126C underoperator control or under computer control in the coordinated mode.Generally, the respective generators are sequenced throughsynchronization by switching actions applied to the synchronizer inputsand outputs.

Once the plant reaches hot standby and either gas turbine or both gasturbines have been started, the steam turbine can be started whenminimum steam supply conditions have been reached. Thereafter, theturbines are accelerated to synchronous speed, the generators aresynchronized and the fuel and steam valves are positioned to operate theturbines at the demand load levels. The manner in which the controlsystem 50 is configured and the manner in which it functions throughoutstartup and loading depends on the selected plant mode and the selectedor forced plant configuration and the real time process behavior.

D. Control System Hardward Components and Organization

Referring now to FIG. 4, there is seen a block diagram of the majorhardware portions of control system 50 of this invention, in combinationwith the plant equipment. The BTG board 103C is theboiler/turbine/generator board which comprises push buttons for operatorcontrol of those units, and is a portion of the overall operator panel102C. Interfacing with the BTG board is the digital control center (DCC)and the analog control center (ACC), which control centers track eachother. The ACC comprises analog control circuitry and runback andpermissive circuitry, as well as input/output circuitry, switchingcircuitry, etc. Both of these control centers output to NHC cards 69,which are a form of digital/analog interface. The output of the NHCcards is connected to a further portion of the ACC, which outputs to theequipment actuators which in turn control the plant equipment. Feedbacksignals of sensed equipment conditions are returned to the BTG board,the DCC and the ACC.

Referring to FIG. 5, there is shown a more specific block diagram of thedigital control system organization for the digital portion of thecontrol system of this invention. Interrupt wiring 55, e.g., contactclosure outputs, are connected to monitor-executive programs 56. Theconnected signals include service request interrupts (SRI) and externalinterrupts (EI). The monitor-executive programs comprise an operatingsystem which sets the hierarchical system for the digital computeroperation, and comprises a standard package. The output from block 56goes to block 58, designated "Paceware Operating Programs". Theseprograms are suitably the Westinghouse PROGEN package, which performsadditional functions similar to the monitor/executive programs, such asanalog input scanning, and contact output bookkeeping. Every second anoutput is provided from block 58 to a visual display program 59, whichprovides for digital displays.

The BTG board push button wiring 103C inputs to the BTG board decodeprogram 60, which provides appropriate digital signals representative ofthe BTG board inputs. Program 60 interacts with the BTG board systemblock 61, which makes responses to the signals from block 60 and passesthem to logic system 67 and the control system 50A, where controlsignals are produced in accordance with programmed subroutines.Additional plant wiring 65 is scanned by the digital scan program 66,which provides inputs to logic 67 as well. Logic system 67 providesoutputs to the BTG lamps 68, and inputs to control system 50A. System50A is bid every second by the auxiliary sync program 62, which programalso periodically bids the analog scan program 64, which derives signalsfrom the plant instrumentation 63 and passes them through to the controlsystem 50A. The output of system 50A is directed to the NHC cards 69,which provide appropriate analog outputs for transmission to equipmentactuators.

Still referring to FIG. 5, as well as Table I set forth hereinbelow, themonitor executive programs establish a control computer priority for alldigital computer operations. The priority assignment for the subjectinvention is set forth in Table I, level F being the highest priority,and level 0 being the lowest priority. The three control function levelsare 9 (coordinated control), 8 (GT control) and 7 (ST control), thecoordinated control being the highest priority.

                  TABLE I                                                         ______________________________________                                        CONTROL COMPUTER PRIORITY ASSIGNMENT                                          Level       Function        Frequency                                         ______________________________________                                        F        Contact Scan/Stop-Initialize                                                                     Demand                                            E        Auxiliary Sync     0.1 second                                        D        Panel (BTG Board)  Demand                                            C        Analog Scan        1/30 second                                       B        GT Logic           Demand                                            A        ST Logic           Demand                                            9        Coordinated Control                                                                              1 second                                          8        GT Control         1/30 second                                       7        ST Control         1 second                                          6        Visual Display     1 second                                          5        Data Link          1 second                                          4        Programmers Console                                                                              Demand                                            3        Simulation         1 second                                          2        Bootstrap          Demand                                            1        Loader             Demand                                            0        Timing Task        Free Timer                                        ______________________________________                                    

E. Combined Cycle Digital Control System Hierarchy

Referring now to FIG. 6, there is shown a block diagram, representingthe digital control system hierarchy of the system of this invention. Atthe top level is shown the coordinated control system, which as statedhereinabove is the highest level of operation. When in coordinatedcontrol, the system maximizes the extent of digital control. The secondlevel of digital control is in operator automatic, illustrated by thethree blocks titled "AUTOMATIC CONTROL SYSTEM" for each gas turbine andassociated HRSG, as well as the steam turbine. Thus, when any one of thethree turbines, or all three of them, are under the operator automaticmode of control, certain control functions (at the 8 level) are carriedout by the digital computer. As will be expanded upon hereinafter, anycombination of the three turbines and two afterburners can be in thecoordinated control mode, and the others in automatic control. FIG. 6also shows three clocks designated "manual tracking system", with arrowsshown going to and from each block corresponding to the three turbines,indicating that the manual and digital control subsystems track eachother, so that transfer from one to the other for any one or all of theturbines is bumpless.

F. Control System Functional Organization

Referring now to FIG. 7, there is shown in greater detail the nature ofthe control system 50. As indicated in FIG. 7, the control system 50includes a load demand unit 71 which generates an electrical signalrepresenting the desired total plant output power level. Unit 71includes means for enabling the plant operator or engineer to change theload demand signal to any value he may wish to select. The load demandsignal from unit 71 is supplied to a load reference logic unit 72 whichalso receives the low level output signals from the megawatt sensors 80,81 and 82. Load reference logic unit 72 produces on its output line aload reference signal which, depending on the operating mode, may or maynot be the same as the load demand signal from unit 71. The loadreference signal from unit 42 is the computed signal which directs themanner in which the system will move to the load demand, and is suppliedto a load distribution control unit 43.

Load distribution control 43 sends appropriate individual demand signalsto a first gas turbine control 104C which drives the first gas turbinefuel valve 14, a second gas turbine control 106C which drives the secondgas turbine fuel valve 24, a first afterburner control 112C which drivesthe first afterburner fuel valve 19, a second afterburner control 114Cwhich drives the second afterburner fuel valve 29, and a steam turbinecontrol 110C which controls the steam turbine throttle valve 35 and thesteam bypass valve 45. Temperature indicating signals from temperaturesensors 86, 84, 85 and 83 are supplied to the gas turbine control 104C,the afterburner control 112C, the gas turbine control 106C and theafterburner control 114C, respectively. Signals from temperature sensors86 and 85 are also supplied to the load distribution control 73. Variousoperating modes for the load reference logic 72 and the loaddistribution control 73 are established by signals supplied to theseunits by a selection system 75.

Still referring to FIG. 7, it is to be noted that the functions ofblocks 72, 73, 75, 104C, 106C, 110C, 112C and 114C are carried out bycombined digital and analog circuitry, i.e., by the ACC and DCC incombination. The exact control configuration depends upon the selectedmode of control, as has been discussed. The selection system 75 includesan array of backlighted push-button switches and status lights which aremounted on operator panel 102C. The specific switches, instruments andcontrol devices have not been illustrated for the sake of simplicity.

COORDINATED CONTROL SUBSYSTEM Coordinated Control Bidding Sequence

Referring now to FIG. 8, the coordinated control bidding sequence isshown in block diagram form. It is to be noted that each block of thebidding sequence is at a 9 level, and that as indicated in Table I thisis the highest periodically bid level of the computer control system.The 9 level is bid every one second, subject to interruption by a higherlevel demand bid. The first program, plant trouble monitor 900B,establishes conditions for providing annunciator and typewritermessages. This is primarily a bookkeeping program. The next bid programis 900A, titled Boiler Startup Monitor, which scans logic statesrepresentative of a plurality of HRSG conditions, forms bookkeeping onthe system logic states, and sets start and stop contact outputs. Thisprogram bids program 9009 just before exit.

Program 9009, the coordinated turbine startup monitor, monitors logicstates of the three turbines, performs bookkeeping functions upon theprocess logical variables, and produces annunciator and typewritermessages. Just prior to exit, this program bids program 9008. Program9008 is the select operating mode program, and is the primary place inthe coordinated control 9 level bidding sequence where control modebookkeeping and logic functions are performed. The program deermineswhether any given ones of the turbines and afterburners are incoordinated control, and according to this determination logic steps areperformed to provide for the selected coordinated control. This programalso includes carrying out of the REFDMD algorithm which produces thereference signal V6992, which in FIG. 7 is the output of load referenceblock 72. Just prior to exit, this program bids program 9007.

Program 9007 is the steam turbine load distribution program, wherein itis determined whether the steam turbine is to load or unload. When thesteam turbine unloads, it unloads to a minimum target load, where theturbine trips. If the steam turbine is in coordinated control and is tobe loaded, this program calls for loading to the maximum power level, or120 MW. As noted previously, when operating the steam turbine valves arenormally called upon for wide open operation. The rate of opening, orrate of loading, is a function of curves incorporated into a lower levelcomputer program, the 700A package. At the end of program 9007, program9005 is bid.

Program 9005 is the gas temperature setpoint supervision program whichis discussed functionally in connection with FIG. 9, the coordinatedcontrol functional diagram. This program generates the setpoint inputsignal for the HRSG/AB control programs, as well as the total GT loaddemand signal. Just before exit, this program bids program 9006.

Program 9006 is the gas turbine load distribution program, which checksthe logical states to determine which gas turbines are in coordinatedcontrol and the state of the gas turbine breakers, and generates demandand rate signals for each gas turbine control program (which latterprograms are carried out at the 8 level). Just prior to exit, thisprogram bids program 9003, which in turn bids 9002 upon completion.Programs 9003 and 9002 are the HRSG/AB control programs, for boilers 1and 2 respectively. These programs bookkeep respective logic conditionsand pass through the gas temperature setpoint signals to the respectiveafterburners when appropriate. They also introduce a different setpointin the absence of detected afterburner flame, track the setpoint when inmanual control, and perform bookkeeping functions corresponding to thestatus of the HRSG/AB control.

In the discussion to follow of certain digital program packages of thecontrol system of this invention, reference will be made to Process RealVariables, Process Logical Variables, and system Constants. These areset forth in the Glossary, Appendix I.

Coordinated Control Functional Arrangement

Referring now to FIG. 9, there is shown a functional diagram of thecoordinated control system of this invention. The nine sub-programs asshown in the coordinated control bidding sequence, FIG. 8, are all shownin this diagram, with sub-program 9005 in particular being shown inexpanded functional form. The plant trouble monitor portion, 900B,receives logic state inputs from the plant and produces annunciator andtypewriter message outputs when predetermined troublesome conditions arefound. Likewise, turbine startup monitor 9009 receives logic stateinputs, and produces annunciator and typewriter messages during theautomatic turbine startup portion of plant control. Boiler startupmonitor portion 900A receives logic state inputs concerning thecondition of the boiler, and produces start and stop outputs which areconnected to plant contacts for direct control of the boiler. Thesethree programs perform a variety of bookkeeping programs essentiallyoutside of the main control loops.

The select operating mode function 9800, which is performed withinprogram 9008, is seen to interface with the steam load distributionprogram 9007 and the gas temperature setpoint supervision program 9005.At block 9800, the control system checks inputs from logic states todetermine exactly what operating mode is called for, and establishescorresponding logical conditions for carrying out control in theselected operating mode. Demand signal V6993 is connected through to theREFDMD block 9802, for calculating the reference signal V6992. Demandsignal V6993 is also stored for use in the steam load distributionprogram 9007, which is bid next in the bidding sequence. The functionsperformed in supervision program 9005 include generation of total GTload demand signal V6998, which is stored for use in the gas turbineload distribution program 9006, and the afterburner setpoint signalV6991, which is utilized in HRSG/AB control programs 9003 and 9002.Thus, program 9005 is seen to generate process real variables which areemployed in the same 9 level digital computer cycle of calculations forgas turbine load distribution and HRSG/AB control respectively.

The steam turbine load distribution program 9007 checks to see whetherthe steam turbine is in coordinated control, and whether otherconditions are present allowing it to proceed in coordinated control.This program checks the demanded plant load and determines whether thesteam turbine is to load, unload, trip out or hold. This programoperates in conjunction with the 7 level ST/BPV control program, whichlatter control program receives the steam load demand signal fromprogram 9007, when the steam turbine is in coordinated control. When thesteam turbine is not in coordinated control, program ST/BPV Controlreceives its input demand signal from the automatic control softward.

Program 9006, GT Load Distribution, performs additional logic evaluationfunctions concerning the gas turbines, and generates demand and ratesignals for use in the GT1 and GT2 control programs at the 8 level. Whenboth gas turbines are in coordinated control they receive identicalsignals from program 9006. If only one gas turbine is in coordinatedcontrol, it continues to receive its setpoint signals from 9006. Eithergas turbine which is operating not in the coordinated control mode, butrather in the operator automatic mode, is controlled from the blocksdesignated GT1 and/or GT2 control, operating at the 8 level, with theinput demand/rate signals for these programs being derived throughoperator input and associated digital and analog control circuitry.

Control programs 9003 and 9002 operate to provide AB setpoint signalsonly when the corresponding afterburners are in coordinated control.Under these circumstances, these programs receive a setpoint signal,V6991 from program 9005, which is of course identical for the twoprograms when both afterburners are in coordinated control. There is nolower level digital computer control for the afterburners, which areeither in coordinated control or operator analog control.

Referring further to the 9005 portion of the functional diagram, themegawatt signals for the three turbines are summed at 6999, generating aplant megawatt signal V6999 which is subtracted from the referencesignal V6992 to provide an input error signal to controller 9520.Controller 9520 provides proportional plus integral control, in additionto high and low limiting, producing an output signal V6975. The outputerror signal is utilized in two control paths, namely the control pathwhich produces the input to GT Load Distribution program 9006, as wellas the control path which produces the input to the afterburner programs9002 and 9003.

The reference signal V6992 from block 9802 is operated upon bysubtracting therefrom V3104, reflecting the steam turbine megawatts,which difference is then summed with the controller signal V6975 afterit has been multiplied by a scaling factor at block 9536. This summationsignal, V6998, is stored for use upon execution of GT load distributionprogram 9006.

The gas turbine exhaust signals, V1961 and V2961, are compared at block9510, with the larger signal being selected for the input to functiongenerator block 9515. A block 9515, a feed-forward signal V6990 isproduced, as is described more fully hereinbelow. This signal is summedwith signal V6972, which in turn is derived by scaling the controlleroutput signal V6975 at block 9535. The output of summer 9516 is operatedon at block 9517, where there is subtracted a signal V6989 which isderived in accordance with an evaluation of the stop, unload and triplogic inputs to block 9521. The output of 9517 is low limited atfunction block 9518, before being stored for use in programs 9002 and9003.

An overview of the coordinated control functional diagram, as seen inFIG. 9 provides insight into the operation of the control system atdifferent control levels when in the coordinated mode, as well as whenin the operator automatic mode of operation. As seen in the functionaldiagram, the 9 level functions are fully carried out whenever the plantis in coordinated control. However, when in operator automatic control,the demand/rate signals are generated in the 8 level and 7 levelprograms. Further, during such operator automatic mode of operation, thedigital computer is continuously operating, checking logic conditions,and tracking the backup control system, as in fact it does in either theanalog operator or manual modes of control. The reference block 9802tracks plant megawatts through track auto block 9801, and as discussedin detail hereinbelow control programs 9002 and 9003 track theafterburner analog control signals when they are not in the coordinatedmode of operation. The control functions carried out at the 8 and 7levels can be carried out independently when the plant is in operatorautomatic control, or can be carried out in conjunction with the 9 levelcontrol functions when the plant is in the coordinated mode of control.

Coordinated Boiler Startup Monitor Flow Diagram, 900A

Referring to FIG. 10, there is seen a flow diagram of program 900A,which provides bookkeeping for the coordinated boiler startup. At thestart of the program, process logical states for the HRSG1 COORD, normalstop, and COORD go are monitored. If HRSG1 is not in COORD, the normalstop button has been pushed, or coordinated go has not been pushed, theprogram jumps to point A. If none of these conditions are met, thestatus of the HRSG run contact input is determined at 9A1. If it isfound to be in the run state, it is set false. Effectively the runcontact output has done its job of starting the boiler. If it is notfound to be in the run state, and HRSG1 standby is found to be negativeat 9A2, HRSG1 run contact output is also set false. However, if theHRSG1 run contact input is found to be negative at 9A1, but the HRSG1standby is found to be positive at 9A2, then the HRSG1 run contactoutput is set true, indicating that the boiler is to be placed in therun mode by the boiler startup monitor program. Proceeding to point A,it is determined at 9A5 whether the coordinated demand signal V6993 isless than the coordinated minimum demand load with no afterburner,K6978. If the demand signal is less than K6978, process logic variableL4001, HRSG1 start AB CO, is set false, indicating that afterburner 1 isnot to be started since plant demand does not require afterburners inservice. If V6993 is not less than K6978, it is determined at block 9A6whether GT1 flame is on. If it is not, L4001 is set false. If GT1 flameis found to be on, it is determined at block 9A7 whether AB1 flame ison. If it is, L4001 is set false, but if it is not it is set true. Thus,coordinated boiler startup places the boiler into service. Proceeding topoint B in the 900A program, the same steps are taken for HRSG2,determining whether the HRSG2 run CO logic variable is set true orfalse. Following that, in a like manner as for afterburner 1, the HRSG2start AB CO logic variable is set true or false. Programs 900B and 9009,which are not included in this specification in detailed form, providesimilar bookkeeping functions.

Coordinated Select Operating Mode Program, 9008

Referring now to FIG. 11, there is seen a flow diagram of thecoordinated select operating mode package 9008. At block 980, it isdetermined whether the plant is in coordinated control. If no, betweenblock 980 and exit, for each turbine having its breaker closed, i.e.,supplying load, the megawatts being delivered are added, and thereference variable V6992 is set equal to the calculated total MW for upto three turbines. This represents a portion of the track auto block9801 in FIG. 9. If the answer at block 980 is yes, the plant is incoordinated control and the program proceeds to block 981 to determineif the plant is in normal stop. If yes, logic variable L6602 is settrue. If the plant is not found to be in normal stop, each turbine ischecked to see if it is in coordinated control. If any one of the threeturbines is in coordinated control, but its breaker is not closed, logicvariable L6602 is set false. Otherwise, the variable L6602 is set true,since all three turbines are in coordinated control with theirrespective breakers closed. Proceeding to point A of the diagram, theportion between A and B represents coordinated control responding toplant contingencies by immediately reducing load level at a high rate.Starting at point A, logic variable L6600 is first set false. At block984, it is determined if the HRSG1 plant runback CI is high and HRSG1 isin coordinated control, or if HRSG2 plant runback CI is high and HRSG2is in coordinated control. If yes, the functions of block 987 arecarried out, with the demand variable V6600 being set equal to the plantrunback rate K6996, and logic variables L6601 and L6986 being set true.If, at block 984, the answer is no, the program checks condensate pumprunback at block 985. If this condition is no, the program skips toblock 988, to check the setting of L6602. If L6602 is yes, the variableV6600 is set equal to 1/2 plant maximum demand. Returning to block 985,if the answer is yes, V6600 is set equal to max. plant demand divided by2. Then at block 986 it is determined whether the demand referencevariable V6992 is greater than V6600. If yes, the functions of block 987are carried out. If no, at block 988 the program checks the previouslydetermined logic variables L6602. If this is yes, the program exits toprogram 9007. If no, variable V6600 is set equal to the coordinated loadrate, and logic variables L6601 and L6986 are set true.

At block 9802, the REFDMD function is performed, this being thedetermination of the reference signal V6992 as a function of demandsignal V6993 (see also FIG. 9, coordinated control functional diagram).The generation of the reference signal provides a control signal formoving the plant, or any part thereof, toward the demand. This is thesame function as is carried out in block 72 of FIG. 7, and is basicallya ramp signal, ramping reference to demand. The rate of the ramp isentered by the operator, or by determined dynamic conditions as in block987. It is to be noted that the reference signal is used for bothloading and unloading, and is subject to the special conditions inputtedto the program, as shown in block 9802.

Following this, it is determined whether the system is in either normalstop or coordinated unload. If no, the program exits to the nextprogram. If yes, it is determined whether the AB computer setpoint isless than or equal to the AB MIN setpoint, K6981. If no, the programexits, but if yes, it is determined at 9812 whether HRSG1 is incoordinated control. If no, it is then determined whether HRSG2 (L6073)is in coordinated control, and if neither are in coordinated control,the program goes to point D. If HRSG1 is in coordinated control, thesystem compares the AB1 setpoint, at the output of the NHC card, with ABMIN setpoint K6981. If it is greater, i.e., not less, the program goesto C; if less, it remains to determine if HRSG1 RUN CI is yes, and notL1082 is yes, and normal stop is yes. If this is so, HRSG1 is set onstandby by making contact output L4003 true. This represents coordinatedcontrol stopping the afterburners and boilers. It is next determinedwhether AB1 flame is on. If it is, the system sets the AB1 STOP CO true.At point C, the same logic steps are carried out for HRSG/AB2. At pointD, the system examines GT1 COORD, GT2 COORD, GT1 4x (an intermediatecontact that determines whether GT1 is running), and GT2 4x. If one ofthe shown logic conditions exist, reference variable V6992 is set equalto zero, as the last step in a normal plant shutdown by coordinatedcontrol.

Coordinated Steam Load Distribution Program, 9007

Referring now to FIG. 12, showing program 9007, process logic variablesL6980 and L6990 are initially book kept by being set equal to the falsecondition. At block 971, the control system determines whether the steamturbine is in coordinated control and the breaker is closed, whichconditions must be met in order to proceed in coordinted control. If theanswer is no, logic variable L3299 is set false, meaning that the steamturbine trip status is reset, whereby coordinated control has stoppedthe steam turbine in normal plant shutdown. However, if the steamturbine breaker is closed and the turbine is in coordinated control, thecontrol program next determines, at 972, whether the normal stop buttonhas been pushed by the operator. If the answer is yes, the programbranches to block 976, sets L6980 true, such that the steam turbine isin coordinated unload status, and sets the steam coordinated demandprocess real variable V3979 equal to minimum steam load, which isessentially 10 MW. By this procedure, the system proceeds to unload thesteam turbine. However, if the answer at block 972 is no, V3979 (steamcoord demand) is set equal to the max load, at block 973. At block 974,the system determines whether the steam turbine is on coordinated hold,and if yes the program exits without doing anything further. If in hold,this means that the operator has placed the system in a hold state suchthat the turbine won't move until he pushes a GO button, even though thedemand has been set. If it is determined that the steam turbine is notin hold, the system checks to see whether coordinated demand is lessthan the megawatt level requiring afterburner firing, as established byK6978. If the answer is no, the program exits. If the answer is yes, thecoordinated afterburner unload process logic variable L6990 is set true,and following this the aforementioned functions of blocks 976 arecarried out.

Referring now to FIG. 13A, there is seen a flow diagram of a portion ofprogram 700A, one of a plurality of programs at the 7 level. Thisprogram is executed in both the coordinated control mode of operationand the operator automatic control mode of operation. When incoordinated control, program 700A operates in conjunction with program9007, the two programs in combination providing the load demand and loadrate setpoint signals which control the two steam turbine valves as wellas the steam bypass valve. In program 700A, which is part of the steamselect operating mode subprogram at the 7 level, the system firstdetermines that the steam turbine is in coordinated mode and is not inmanual control, that there is no throttle pressure runback, and that thesteam breaker is closed. Following this, at block 7A1, it is determinedwhether the system is in coordinated steam unload. If no, meaning thatthe steam turbine is in coordinated load mode of operation, the loadfunction is generated in block 72A. Referring to FIG. 13B, there isshown a curve representing the load function generated in block 7A2, theinput being V3969, throttle pressure state, and the output being anintermediate variable V3958. V3969 is determined as a function of sensedsteam throttle pressure, and is derived in another sublevel program notshown. It is to be noted that V3958, which is representative of loadingrate, actually goes negative, thus calling for unloading, if thevariable V3969 drops below a predetermined level. The stored arraydetermining the curve of FIG. 13B is designated as K6870 and may, ofcourse, be adjusted to adapt to the steam turbine in use. Followinggeneration of the variable in block 7A2, the demand rate signal V3978 isset equal to V3958, certain other bookkeeping functions are performedand the next sublevel is bid before exiting. If the answer is yes atblock 7A1, there is performed at block 7A3 an unload functioncorresponding to the load function of 7A2. The unload function ispresented in FIG. 13C, the input variable again being V3969, the outputvariable V3957, and the stored array which determines the relationshipbetween input and output being designated by K6850. Upon determinationof the intermediate unload function at block 7A3, the system determines,at block 7A4, whether the steam turbine megawatts being produced areless than the minimum load, K3991. If no, such that unloading is tocontinue, the demand rate signal V3978 is set equal to the computedvalue of V3957. If the answer at 7A4 is yes, it is next determined, at7A5, whether a normal stop is called for. If the answer is yes, thesteam coordinated trip logic variable is set true before bidding thenext sublevel. This represents coordinated control stopping the steamturbine as part of normal shutdown of the plant. If a normal stop is notcalled for, the demand rate signal V3978 is set equal to zero beforebidding the next sublevel.

Coordinated Gas Temperature Setpoint Supervision Program, 9005

Reference is made jointly to FIGS. 14 and 15, the functional diagram andflow chart respectively for program 9005. In the first line of the flowchart, FIG. 15, the plant megawatt (MW) signal, V6999, is obtained. Thissignal is first set to zero (block 9550) and then is obtained bytotaling the received MW signal for the steam turbine and each of thetwo gas turbines, in blocks 9552, 9554, and 9556. If any of therespective turbine breakers are not closed, the corresponding turbine MWsignal is set to 0. Next, the total GT load demand signal, V6998, isobtained. This is obtained by first setting it equal to the referencesignal, V6992, which is obtained from the 9008 program, and then (atblock 9556), subtracting the steam megawatts (V3104). Note that later inthe program, at block 9575, V6998 is further modified by adding to itV6971, which is the scaled plant megawatt feedback signal from thecontroller. While these functions are being performed, the last MWreading is set for GT1 (V6978); GT2 (V6977); and ST (V6976).

The flow chart portion between A and B relates to the stop, unload andtrip logic (block 9521 in FIG. 9), producing variable V6989. The firstlogical step, at block 9502, is to determine whether the system callsfor either a normal stop, or for the combined condition of coordinatedunload and not hold (block 9501) and also afterburner flame on in eitherunit (block 9507). If the first logical question is answered yes atblock 9503, logic variable L6600 is set positive, which has the effectof saying to the system that you want to bring the afterburner down tominimum load. Accordingly, V6989, which is substracted from the gastemperature setpoint signal at 9517 on the functional diagram, must goto the value of afterburner temperature span max-min, such that thedifference out of 9517 (FIG. 14) goes to simply AB min. This function isperformed at block 9507, where V6601 is set equal to K6980 minus K6981.If the first logic statement is found to be NO at block 9503, meaningthat the system doesn't want to unload, the next logical question at9504 is whether the system is in hold. If yes, V6989 is not changed, sothe program skips to point B. If the answer is no, meaning that thesystem is not in stop, unload or hold, V6989 is set equal to zero at9506, such that there is no effect upon the generated setpoint signal.For any change in V6601, from block 9506 or block 9507, the change isramped in block 9505, where the output is V6989, and the input is V6601.The ramp rate is set by constant K6998, and the output reference signalis stored.

Referring now to path B through C, this comprises the MW controllerlogic. The first logic condition is established as represented by fourinputs to OR gate 9529. If any one of these input conditions is met, apositive logic signal L6600 is determined at block 9530, signifying thatthe system wants the MW feedback signal to be zero. Accordingly, in thiscircumstance, the input signal V6973 to the controller is set to zero(block 9532), and the controller output is ramped to zero (at block9531). Note that in block 9532 both the last MW input signal and the MWintegral signal are set to zero. With these two inputs set to zero, themegawatt controller output (V6975) is ramped to zero.

If, at block 9530, L6600 is found negative, such that the first logiccondition is not met, but the system is determined at 9531 to be onunload and hold, the program does not repeat the controller calculation,but simply holds the controller output, skipping the block 9540. If theanswer at block 9531 is no, such that the system is not on hold, theprogram performs the normal controller function. First the coordinatereference signal V6992 is adjusted at 9533 by substracting the plantmegawatt signal V6999, to get a new MW error value of V6600. This value,as well as the last previous input to the controller, are operated on atcontroller block 9520 to get the controller output V6975. The controlleroutput, for any logic condition, is then scaled by K6991 to get V6972,and is multiplied by K6982 to get V6971 (at block 9540). See also FIG.9.

Still referring to FIG. 15, in the path from C to D, the program derivesa gross feedforward setpoint from the gas turbine exhaust temperatures.First, the two exhaust temperature input signals are compared, at block9541, and in blocks 9542 or 9543, variable V6600 is set equal to theselected higher temperature signal. The feedforward signal, V6990, isdetermined in function block 9515, as a function of input V6600 andstored array K6890 (see FIG. 9). Array K6890 is suitably set to providea linear increase in the feedforward setpoint signal V6990 as a functionof GT exhaust temperature between 450° F. and 900° F., with thegenerated setpoint signal suitably being held constant outside of suchtemperature range. However, the range may be varied, and the changewithin the range may be adjusted to provide other than a linearincrease. The important feature of this portion of the control system isto provide a feedforward signal which is not merely proportional to theexhaust temperature, but which provides a measured interface between theGT condition and the following condition of the steam turbine, i.e.,which acts to control the manner of afterburner response so as tooptimally contribute to the coordinated steam turbine followingoperation.

In block 9575, the output of function block 9515 is added with thescaled controller signal V6972, and there is then substracted thestop/unload signal V6989. Additionally, the total gas load signal,V6998, which is inputted to program 9006, is obtained by adding to thepreviously computed value of V6998 the latest value of the feedbackcontroller signal V6971.

In the path of program 9005, D to exit, the program checks to see thatthe gas temperature setpoint signal V6991 doesn't drop below the AB minsetpoint when either GT is on coordinated control. This corresponds tothe function performed at block 9518 in the 9005 functional diagram. Theprogram then bids 9006 and exits.

Coordinated GT Load Distribution, 9006

Referring now to FIG. 16, there is shown the flow diagram for program9006, the coordinated GT Load Distribution program. In this program, itis determined which, if either, of the gas turbines is in coordinatedcontrol, corresponding demand and rate signals are determined for eachgas turbine which is in coordinated control, and the tracking functionbetween the automatic and coordinated control is performed. Starting atblock 961, the two GT load rate signals are book kept by being set tothe coordinated load rate. In block 962, it is determined whether theGT1 breaker is closed. If yes, the program goes directly to block 963 todetermine if the GT2 breaker is closed, and if not, the program proceedsto block 964, where also it is determined if the GT2 breaker is closed.At block 964, if GT2 breaker is not closed, such that neither breaker isdetermined to be closed, the program bids program 9003 and exits. If theGT2 breaker is found closed, it is determined at block 965 whether GT2is in coordinated control. If yes, variable 6001 is set equal to the GTload demand, which is obtained from program 9005, and the tracking biasis set equal to zero. If no, the tracking bias is computed at block 966,and ramping is provided for bumpless transfer between coordinated andautomatic control. The tracking bias is established as a function of thedifference between the digitally computed automatic and coordinatedreferences.

Proceeding to point A in the flow diagram there are performed betweenpoints A and C the same functions as described hereinabove, for thesecond gas turbine. At point B, to which the program proceeds if bothGT1 and GT2 are in the load mode, i.e., both breakers are closed, it isdetermined at 970 whether GT1 is in coordinated control. If yes, it isdetermined at block 971 whether GT2 is in coordinated control. At block973, when GT1 is found not to be in coordinated control, it isdetermined whether GT2 is in coordinated control, and at block 974, whenGT1 is coordinated it is determined if GT2 is coordinated. Thedetermination of what combination of gas turbines is in coordinatedcontrol leads to four function blocks 975-978, in which processvariables are determined as a function of the combination of gasturbines in coordinated control. Block 975 is the function block whereboth turbines are in coordinated control; block 976 where only GT1 is incoordinated control; block 977 where GT2 alone is in coordinatedcontrol; and block 978 where neither gas turbine is in coordinatedcontrol.

Referring to block 975, where both GTs are in coordinated control, thetracking biases for each are set equal to zero, the load rates are setequal to 1/2 total GT coordinated low rate, and the variables V6600 andV6601 are set equal to 1/2 total GT load coordinated load reference. Inblock 976, variable V6600, the GT1 demand variable, is set equal to GTdemand minus GT2 MW, and the GT1 tracking bias is set equal to zero. TheGT2 demand variable to set equal to 1/2 the GT demand, and the GT2tracking bias is determined. Likewise, in block 977, the GT2 demandvariable is set equal to GT demand minus GT1 MW, and GT2 tracking biasis set equal to zero. The GT1 demand variable is set equal to GT demanddivided by 2, and the GT1 tracking bias is determined. In block 978, thetracking bias is determined for each GT, and each GT demand variable isset equal to GT demand divided by 2.

Following the functions performed in the selected one of blocks 975-978,the GT1 tracking function is carried out in ramp block 980, and the GT2tracking function is carried out in ramp block 981. In block 982, GT1demand is determined as the GT1 demand variable plus GT1 tracking rampoutput, and GT2 demand is determined as the GT2 demand variable plus GT2tracking ramp output. The program then bids program 9003 and exits.

Referring now to FIGS. 17 and 18, there are shown the functional diagramand flow chart respectively of the HRSG/AB control system. As notedpreviously, there are respective packages 9003 and 9002 for HRSG1/AB1and HRSG2/AB2 respectively . FIGS. 17 and 18 are presented correspondingto HRSG1/AB1, it being understood that the same diagrams exist forHRSG2/AB2 with corresponding different process real and process logicalvariables.

Referring first to FIG. 17, the functional diagram for the HRSG/ABcontrol, the setpoint signal V6991 is inputted to block 931, where it isdetermined whether afterburner 1 flame is on. If it is, setpoint V6991is passed straight through. However, if the afterburner flame is foundnot to be on, a different setpoint signal is generated at block 932, inaccordance with stored array K6860. The input to array 6860 is V1961,the gas turbine 1 exhaust temperature. This array is, it is to be noted,different from the array utilized for generating signal V6990 in program9005, the difference corresponding to the desired difference setpoint inthe absence of sensed afterburner flame. The ramp function at block 934normally passes the V6600 signal from block 931, but ramps to the newoutput which occurs when the condition of afterburner flame is detectedto have changed. The output of ramp 934, designated as V6601, isinputted to sumation block 935, to which is inputted a tracking signalV4995, the derivation of which is discussed hereinbelow. Next, it isdetermined, at block 936, whether the afterburner is in the coordinatedor analog mode. If in analog, certain logic functions designated "readylogic" are performed in block 937. These functions are bypassed if theafter burner is in coordinated mode. Next, at block 938, certain statuslamp logic functions, specified in the flow chart of FIG. 18, areperformed, following which the V4997 signal is processed into thenecessary bit pattern in output function block 939. This is a step inpreparation for the operation at block 9310, where the digital outputsignal is processed in the nuclear hybrid coupler, which is a printedcircuit card which couples the digital circuitry to the analogcircuitry. The NHC card is disclosed and described in the referencedapplications. The output from block 9310 is V4998, which is theresultant gas temperature setpoint signal which is fed to theafterburner 1, as well as to the following analog circuitry of theHRSG1. This signal is fed back to difference block 9315, to determine adifference signal for tracking between coordinated control and manualcontrol. At block 9320 it is determined whether the after burner is incoordinated or manual control, and if in coordinated control the returnsignal is set to zero. If in manual control, the difference signal isfed through ramp function 9321 to block 935. In this fashion, when theafter burner control is returned to the coordinated mode, the differencesignal is maintained at the moment of transfer, providing bumplesstransfer, and then is ramped down to zero. As seen in the flow chart ofFIG. 18, as part of the ready logic block 937 it is determined at block9375 whether V6600 is less than K4997, i.e., less than the tracking deadband. If the answer is yes, the AB1 ready for coordinated controlprocess logic variable is set true, but if no, this logic variable isset false. Since V6600 at this point has been made equal to the absolutevalue of the difference between V4997 and the fed back output signalV4998, it represents an absolute value of the tracking error signal.Thus, as long as this tracking error signal is smaller than thepredetermined constant, the system allows coordinated control, thusavoiding the tracking dead band range.

Coordinated Control Panel Selection

FIG. 19 shows in a portion of the operator panel which provides forselecting or rejecting coordinated control for the plant or anycombination. As indicated in FIG. 19, there is included an array ofbacklighted pushbutton switches and status lights which are mounted on aportion 76 of the operator control panel.

Items in the 90, 93, 95, 96 and 98 series which have suffix letters "a"and "b" are status lights. Items in these series which have suffixletters "c" and "d" are spring-loaded backlighted pushbuttons which arelit when the switch contacts associated with the buttons are closed. Afirst momentary depression of one of these pushbuttons will close itsswitch contacts and the next momentary depression will open its switchcontacts. Items 152-155 are spring-loaded backlighted pushbuttons whichare used in establishing the different operating modes for the controlsystem 50.

Considering the pushbuttons 95c and 95d for GT1, for example, the"coordinated select" button 95c is depressed if it is desired to haveGT1 operate in the plant coordinated mode. If, on the other hand, it isdesired to have GT1 operate in the operator automatic mode, then the"coord reject" pushbutton 95d is depressed. The depressing of eitherbutton 95c or 95d does not cause the actual change in mode. The actualtransfer from one mode to the other occurs when one or the other of modetransfer buttons 152 and 153 is depressed. In other words, when theplant coordinated button 152 is depressed, then all of the individualunits which have their coordinated select buttons switch contacts closedare at that moment transferred to the plant coordinated mode. In asimilar fashion, the depressing of the plant operator pushbutton 153causes a transfer to the operator automatic mode of those units forwhich the switch contacts have been closed by a depressing of theiroperator select pushbuttons. Transfer pushbuttons 152 and 153 areeffective to provoke mode changes in the load distribution control unit73, while mode transfer buttons 154 and 155 are effective to initiatemode changes in the load reference logic 72.

Considering now the status lights having the suffix a and bdesignations, the "redi" light having the "b" designation is lit if theparticular unit in question is in the operator automatic mode and is ina ready condition or proper condition to be transferred to the plantcoordinated mode. If then the coordinated select button "c" for thatunit is depressed to light same and if the plant coordinated transferbutton 152 is thereafter depressed, the "redi" light "b" goes out andthe coordinated light "a" comes on. The lighting of the coordinatedlight "a" indicates that the transfer to the plant coordinated mode hasbeen accomplished and that such unit is now operating in such mode.Conversely, if the "redi" light "b" is lit, then the unit in question isoperating in the operator automatic mode. Remaining portions of thecontrol panel, not shown, provide similar switching into and out of theoperator analog and manual modes of operation.

Reference is made to FIG. 20, which is a diagram of a 9-level programfor going into or out of coordinated control. This program is usuallyinitiated at operator request, as by pushing the buttons of FIG. 19. Incertain cases, it may be bid from other prior-bid programs. The diagramillustrates the logic followed for setting a large number of the logicvariables which are utilized in coordinated control programs. At blocks9F1-9F8, 9F20-9F22 and 9F25-9F27, logic states are established anddecisions made concerning establishment of logic states. Also, notshown, the digital control computer contains a large number of othersubroutines for interfacing between the coordinated control software andthe BGT Board, and for monitoring plant contact inputs, whichsubroutines are operated by demand, upon interruption, or when one ofthe coordinated control select or reject buttons is pushed.

In review, there is disclosed a digital computer control system, adaptedto operate cooperatively with a back-up analog control system, thedigital control system having a plurality of control levels, and furtherhaving the flexibility of controlling different configurations ofturbines and afterburner elements at given ones of the different controllevels. The highest level of control is coordinated control, in whichmode the plant can be loaded when the appropriate turbine breakers areclosed. The digital computer provides for operation in the coordinatedcontrol mode, whereby any combination of the three turbines and twoafterburners can be put in coordinated control, subject to the priorcondition that the respective turbines have been synchronized and theirbreakers are closed, i.e. they are carrying load. Each of the digitallevels, 9, 8 and 7, have a select operating mode package for carryingout the selection process. At the 9-level of operation, program 9008 isthe select operating mode package which checks logic conditions and setslogic variables for use in the remaining 9-level packages. Likewise, theST/BPV control package has a plurality of programs, one of which is theselect operating mode, and each of the two GT control packages likewisehas a plurality of programs, one of which is the select operation modeprogram. Thus, every time that the digital computer sequences throughthe various priority levels, at each level the logic conditions arechecked to determine which elements are in what mode of control.Reference is made to the aforementioned prior applications which areincorporated by reference, wherein there is a further discussion of theelements of the ST/BPV control block package, the GT1 and the GT2control packages.

It is to be noted that whenever the digital control computer 58G isoperating, all of the various periodically bid levels are sequenced,i.e. first the 9 level is sequenced, then the 8 level is sequenced, andthen the 7 level is sequenced. This statement is true, no matter whatmode of plant control has been selected. If the plant is in coordinatedcontrol, the 9 level programs are carried out, with the controlfunctions being fully determined for each turbine which is incoordinated control. If the plant is in operator automatic control, orany given turbine is in operator automatic control, the 9 level programsare still carried out, but the corresponding sequential steps aremodified subject to the logic conditions, such that the 9 level programseffectively skip many of the control functions and bid to the nextlevel. The 8 and 7 level programs are sequenced in a similar fashion, inboth of the digital computer modes.

It is to be noted that the boiler control functions, as illustrated inFIG. 3, are analog controlled by the analog control center, from standby to full load. However, the gas temperature setpoint (afterburner)control signals generated in programs 9002/9003 are also connected tothe boiler control circuitry, for supervision of the boiler controloperation.

When in plant coordinated control, both gas turbines are operatedtogether. Start-up sequencing of the gas turbines is done in the GT1 andGT2 control packages, respectively. These packages generate controlsignals, as shown in FIG. 9, which operate the gas turbine valves 14 and24 and the input guide vanes 15 and 25. However, both gas turbines neednot be in coordinated control since separate control packages areprovided for them, such that either GT control package can acceptsignals generated in program 9006, as in coordinated control, or fromoperator inputs, as in the other modes of control. Likewise, when inplant coordinated control both afterburners are operated together, buteach is provided with a separate control package. The ST/BPV controlpackage contains a program for monitoring the steam throttle pressurestate, which is sensed by one of the sensors 101C and communicated tothe control computer, and for generating a steam turbine load ratecontrol signal as a function of such sensed pressure state, whereby thesteam turbine may be loaded automatically, simply by the operatortelling it to go. Afterburner control may be either coordinated oroperator analog. In both cases, both the ACC and DCC generate controlsignals, but when in the coordinated mode the digital signals supplantthe analog signals.

In the coordinated control mode, the two primary control paths are (1)that which generates the gas turbine fuel valve and IGV controls, and(2) the one which generates the gas temperature set point controls, orafterburner controls. Each of these is adjusted by a plant load errorsignal representative of overall plant loading. A primary feature of theafterburner path is the incorporation of the means for generating thefeedforward gas turbine setpoint signal as a function of a stored arraywhich is predetermined in order to maintain afterburner operationbetween a minimum of 750° F. (which is required to obtain a minimumsteam temperature of 700° F.) and a predetermined maximum afterburnerinlet temperature. By precisely maintaining the gas temperature at theinlet to the afterburner, the steam turbine load is maintained in afollowing mode from the gas turbine operation.

    __________________________________________________________________________    APPENDIX I                                                                    PROCESS LOGIC VARIABLES                                                       L1079                                                                             GT1 Flame On CI                                                                              L3183                                                                             ST Hot Standby Monitor                                                        Lamp                                                   L1082                                                                             GT1 4X CI                                                                                    L3184                                                                             ST ACC Monitor Lamp                                    L1090                                                                             GT1 Trip Reset CI                                                                            L3188                                                                             ST Follow                                              L1121                                                                             GT1 Coord. Button Lamp                                                                       L3197                                                                             ST MW Unreliable                                       L1965                                                                             GT1 Auto Sync Reject                                                                         L3225                                                                             Hold/State and Lamp                                    L1966                                                                             GT1 Auto Start                                                                               L3231                                                                             AB/State and Lamp                                      L1971                                                                             GT1 Auto Sync                                                                                L3290                                                                             ST Coord. Button Lamp                                  L1975                                                                             GT1 IGV Manual                                                                               L3299                                                                             ST Coord. Trip                                         L1976                                                                             GT1 Fuel Valve Manual                                                                        L3953                                                                             ST Standby                                             L1994                                                                             GT1 Breaker Flip Flop                                                                        L3954                                                                             ST Hot Standby                                         L1995                                                                             GT1 MW Unreliable                                                                            L3963                                                                             ST Auto Start                                          L2079                                                                             GT2 Flame On CI                                                                              L3965                                                                             ST Auto Sync Reject                                    L2082                                                                             GT2 4X CI                                                                                    L3966                                                                             ST Bkr FF                                              L2090                                                                             GT2 Trip Reset CI                                                                            L3971                                                                             ST Auto Sync                                           L2121                                                                             GT2 Coord. Button Lamp                                                                       L3976                                                                             ST Control Valves                                      L2965                                                                             GT2 Auto Sync Reject                                                                             Manual                                                 L2966                                                                             GT2 Auto Start L3977                                                                             ST BPV Manual                                          L2971                                                                             GT2 Auto Sync  L3994                                                                             AB Hold                                                L2975                                                                             GT2 IGV Manual L3998                                                                             ST Control Valves                                                             Open                                                   L2976                                                                             GT2 Fuel Valve Manual                                                                        L3999                                                                             ST Latch                                               L2994                                                                             GT2 Bkr FF                                                                                   L4001                                                                             HRSG1 START AB CO                                      L2995                                                                             GT2 MW Unreliable                                                                            L4002                                                                             HRSG1 RUN CO                                           L3106                                                                             One Condensate Pump                                                           Runback        L4003                                                                             HRSG1 STANDBY CO                                       L3182                                                                             ST Standby Monitor Lamp                                                                      L4004                                                                             AB1 Stop CO                                            L4005                                                                             AB1 Auto CO    L6014                                                                             Standby Status Lamp CO                                 L4051                                                                             HRSG1 STANDBY CI                                                                             L6015                                                                             Hot Standby Status                                                            Lamp CO                                                L4052                                                                             HRSG1 Trip Contact Input                                                                     L6016                                                                             Run Status Lamp CO                                     L4053                                                                             HRSG1 RUN CI                                                                                 L6019                                                                             HRSG2 Flame Status                                     L4054                                                                             HRSG1 AB Flame On CI                                                                             Lamp                                                   L4055                                                                             HRSG1 Plant Runback CI                                                                       L6023                                                                             AB Min Status Lamp CO                                  L4056                                                                             SH1 Outlet Temp Change                                                                       L6024                                                                             AB Reg Status Lamp CO                                      >7.5° C. F/Min CI                                                                     L6025                                                                             AB Max Status Lamp CO                                  L4061                                                                             HRSG1 Dry CI                                                                                 L6027                                                                             ST Standby Status Lamp                                 L4998                                                                             AB1 Ready For Coord.                                                                         L6028                                                                             ST Hot Standby Status                                  L4999                                                                             New Value Entered  Lamp                                                   L5001                                                                             HRSG2 Start AB CO                                                                            L6033                                                                             ST ACC Status Lamp                                     L5002                                                                             HRSG2 Run CO   L6034                                                                             ACC Hold Status Lamp                                   L5003                                                                             HRSG2 Standby CO                                                                             L6039                                                                             HRSG1 Dry Status Lamp                                                         CO                                                     L5004                                                                             AB2 Stop CO                                                                                  L6040                                                                             Standby Status Lamp CO                                 L5005                                                                             AB2 Auto CO                                                                                  L6041                                                                             Hot Standby Status                                     L5051                                                                             HRSG2 Standby CI   Lamp CO                                                L5052                                                                             HRSG2 Trip Contact Input                                                                     L6042                                                                             Run Status Lamp CO                                     L5053                                                                             HRSG2 Run CI   L6045                                                                             HRSG1 Flame Status                                                            Lamp                                                   L5054                                                                             HRSG2 AB Flame On CI                                                                         L6049                                                                             AB Min Status Lamp CO                                  L5055                                                                             HRSG2 Plant Runback CI                                                                       L6050                                                                             AB Reg Status Lamp CO                                  L5056                                                                             SH2 Outlet Temp Change                                                        >7.5° C. F/Min CI                                                                     L6051                                                                             AB Max Status Lamp CO                                  L5061                                                                             HRSG2 Dry CI   L6065                                                                             Plant Coord.                                           L5996                                                                             AB2 Manual     L6066                                                                             Plant Auto                                             L5998                                                                             AB2 Ready for Coord.                                                                         L6067                                                                             Load Control Button                                                           Lamp                                                   L6013                                                                             HRSG2 Dry Status Lamp                                                     L6068                                                                             GT Button Lamp L6086                                                                             Coord. Panel Coord.                                                           Select ST/State and                                    L6069                                                                             GT2 Coord.         Lamp                                                   L6070                                                                             Coord. Panel Coord. Select                                                                   L6087                                                                             Coord. Panel Ready                                         GT2/State and Lamp ST/State and Lamp                                      L6071                                                                             Coord. Panel Ready GT2/                                                                      L6088                                                                             Coord. Panel Coord.                                        State and Lamp     ST/State and Lamp OR                                                          ST COORD. Reject Lamp                                  L6072                                                                             Coord. Panel OP Auto                                                          Select GT2/State and Lamp                                                                    L6090                                                                             Control Coord. Key-                                        OR GT2 Coord. Reject Lamp                                                                        board Enable CO                                        L6073                                                                             HRSG2 COORD. - HRSG2                                                                         L6091                                                                             Coord. Hold                                                in coordinated control                                                                       L6092                                                                             Coord. GO                                              L6074                                                                             HRSG2 COORD. Select                                                                          L6093                                                                             Normal Stop                                            L6075                                                                             HRSG2 Ready for Coord.                                                                       L6094                                                                             Tuning Switch Lamp                                     L6076                                                                             HRSG2 Coord. Reject OR                                                        COORD. PANEL Op Auto                                                                         L6100                                                                             Tuning Switch CI                                           Select HRSG2/State and                                                        Lamp           L6102                                                                             Control Coord. Key-                                                           board Enable CI                                        L6077                                                                             GT1 Coord.                                                                                   L6104                                                                             BTG Power Fail CI                                      L6078                                                                             Coord. Panel Coord. Select                                                    GT1/State and Lamp                                                                           L6154                                                                             Coord. Reject Ann. CO                                  L6079                                                                             Coord. Panel Ready GT1/                                                                      L6155                                                                             Auto Sync Reject Ann.                                      State and Lamp     CO                                                     L6080                                                                             Coord. Panel Op Auto                                                                         L6157                                                                             Plant Demand Runback                                       Select GT1/State and Lamp                                                                        CO                                                         OR GT1 Coord. Reject Lamp                                                                    L6158                                                                             Plant Load Unit - One                                  L6081                                                                             HRSG1 COORD. - HRSG1                                                          in coordinated control                                                                       L6161                                                                             GT1 Trip Ann. CO                                       L6082                                                                             Coord. Select Button and                                                                     L6162                                                                             HRSG1 Trip Ann. CO                                         Lamp, HRSG1                                                                                  L6163                                                                             GT2 Trip Ann. CO                                       L6083                                                                             Coord. Panel Ready HRSG1/                                                     State and Lamp L6164                                                                             HRSG2 Trip Ann. CO                                     L6084                                                                             Op Auto Select HRSG1                                                                         L6166                                                                             Control Message Ann.                                       Button and Lamp, OR                                                                              CO                                                         HRSG1 OP Coord. Reject                                                                       L6167                                                                             Data Link on AD                                        L6085                                                                             ST COORD.          Converter Fail CO                                      L6980                                                                             COORD. ST UNLOAD                                                                             L6991                                                                             Delay Control Message                                                         Ann. Clear                                             L6982                                                                             Keyboard Valid                                                                               L6995                                                                             Plant Op Auto Button                                   L6985                                                                             Computed GO/HOLD Reset                                                                           Pushed                                                 L6986                                                                             Computed Hold  L6996                                                                             Local Plant Coord.                                                            Button                                                 L6987                                                                             Hold Button Pushed                                                                           L6997                                                                             Data Link Not Operating                                L6988                                                                             GO Button Pushed                                                                             L6998                                                                             A/D Convertor Failure                                  L6990                                                                             Coord. Unload                                                                 (AB Unload?)   L6999                                                                             NORMAL STOP BUTTON                                                            PUSHED                                                 PROCESS REAL VARIABLES                                                        V1104                                                                             GT1 MW         V4998                                                                             AB1 NHC Output                                                                (Set Point)                                            V1961                                                                             GT1 Exh T                                                                                    V5998                                                                             AB2 NHC Output                                         V1978                                                                             GT1 Coord. Load Rate                                                                             (Set Point)                                            V1979                                                                             GT1 Coord. Demand                                                                            V6971                                                                             GT MW Feedback Factor                                  V1992                                                                             GT1 Reference  V6972                                                                             AB MW Feedback Factor                                  V2104                                                                             GT2 MW         V6973                                                                             MW Controller Last                                                            Input                                                  V2961                                                                             GT2 Exh T                                                                                    V6974                                                                             MW Controller Integral                                 V2978                                                                             GT2 Coord. Load Rate                                                                             Output                                                 V2979                                                                             GT2 Coord. Demand                                                                            V6975                                                                             MW Controller Total                                                           Output                                                 V2992                                                                             GT2 Reference                                                                                V6976                                                                             ST Last MW                                             V3104                                                                             ST MW                                                                                        V6977                                                                             GT2 Last MW                                            V3978                                                                             ST Coord. Load Rate                                                                          V6978                                                                             GT1 Last MW                                            V3979                                                                             ST Coord. Demand                                                                             V6985                                                                             GT2 Tracking Ramp                                      V4081                                                                             SH1 Outlet Pressure                                                                              Output                                                 V4995                                                                             AB1 Tracking Ramp                                                                            V6986                                                                             GT2 Tracking Bias                                      V4996                                                                             AB1 Tracking Bias                                                                            V6987                                                                             GT1 Tracking Ramp                                                             Output                                                 V4997                                                                             AB1 Set Point                                                             V6988                                                                             GT1 Tracking Bias                                                                            V6993                                                                             Coord. Demand                                          V6989                                                                             Normal Stop Ramp                                                                             V6994                                                                             Coord. Load Rate                                       V6990                                                                             AB Characterized GT Exh T                                                                    V6998                                                                             Total GT Load Demand                                   V6991                                                                             AB Gas Temp Set Point                                                                        V6999                                                                             Plant MW                                               V6992                                                                             Coord. Reference                                                          CONSTANTS                                                                     K3991                                                                             ST Min Load    K6981                                                                             AB Min Set Point                                       K3992                                                                             ST Max Demand  K6982                                                                             MW/GT Controller                                                              Ranging Gain                                           K4997                                                                             Tracking Dead Bank                                                                           K6989                                                                             GT2 Tracking Ramp Rate                                 K4998                                                                             Tracking Ramp Rate                                                                           K6990                                                                             GT1 Tracking Ramp Rate                                 K4999                                                                             Zero                                                                                         K6991                                                                             MW/AB Controller                                       K5996                                                                             Coord. Min Demand, No AB                                                                         Ranging Gain                                           K6890                                                                             GT Exh T/AB Array                                                                            K6992                                                                             Plant Max Demand                                       K6950                                                                             MW Controller Array                                                                          K6993                                                                             MW Controller Ramp                                                            Rate                                                   K6978                                                                             Min Coord. Load Demand                                                        With No AB     K6996                                                                             Plant Runback Rate                                     K6980                                                                             AB Max Set Point                                                                             K6998                                                                             Normal Stop AB Ramp                                                           Rate                                                   __________________________________________________________________________

What is claimed is:
 1. A control system for an electric combined cyclepower plant having at least a gas turbine and a steam turbine and a heatrecovery steam generator interconnected in a heat cycle to providemotive energy for driving said turbines, means for generating electricpower in response to mechanical drive power from said turbines, saidcontrol system comprising a gas turbine control and a steam turbinecontrol each being a portion of a digital control center and an analogcontrol center, said digital and analog control centers providing meansfor a plurality of control modes at respective different levels ofautomation, the steam turbine control including means for providing atleast two modes of control for automatically loading and unloading saidsteam turbine as a function of sensed steam pressure at a predeterminedpoint in said steam turbine.
 2. The electric power plant as described inclaim 1, wherein said digital control center comprises stored programmeans for carrying out the function of selecting which ones of theturbines of said combined cycle power plant are selected for coordinatedcontrol, coordinating the loading and unloading of said turbines, anddetermining the load and unload rate of said steam turbine as a functionof a predetermined throttle steam pressure characteristic.
 3. Thecombined cycle plant as described in claim 2, wherein the functions ofselecting which turbines are in coordinated control, determiningcoordinated load distribution, and determinating the steam turbine loadand unload rates are carried out periodically and in a predeterminedsequence.
 4. The combined cycle electric power plant as described inclaim 3, comprising means for accepting operator inputs for determiningload distribution to said steam turbine, and wherein said digitalcontrol center stored program means comprises means for automaticallycontrolling the rate of change of load of said steam turbine when saidsteam turbine is or is not under coordinated control, said program meansalso operating on said inputs and automatically determining load andunload rates according to said predetermined function in order to loadand unload said steam turbine.
 5. A control system for an electriccombined cycle power plant having at least a gas turbine and a steamturbine and a heat recovery steam generator interconnected in a heatcycle to provide motive energy for driving said turbines, means forgenerating electric power in response to mechanical drive power fromsaid turbines, said control system comprising:a. means for coordinatinggeneration of load control signals for controlling said gas turbine andsaid steam turbine; b. means for generating steam load rate controlsignals as a selected function of a predetermined steam turbineoperating condition; c. means for controlling said steam turbine as afunction of said coordinated load control signals and said steam loadrate control signal; d. means for providing a plurality of control modesfor operating said coordinating means and said generating means atdifferent levels of automation.
 6. The control system as described inclaim 5, wherein said operating condition is steam turbine throttlepressure, and comprising analog means for measuring said throttlepressure.
 7. The control system as described in claim 6, wherein saidload rate means comprises a first array of stored information from whichsaid selected function is derived during steam turbine loading.
 8. Thecontrol system as described in claim 7, wherein said load rate meanscomprises a second array of stored information from which said selectedfunction is derived during unloading.
 9. The control system as describedin claim 8, comprising means for determining when said steam turbine isloading and when it is unloading, and selection means for selecting oneof said two arrays in accordance with said determination.
 10. Thecontrol system as described in claim 9, wherein said coordinating meansand said steam load rate means both operate in a first of said modes andsaid load rate generating means operate without said coordinating meansin a second mode, and means for selecting one of said modes.
 11. In acombined cycle electric power plant having at least a gas turbine and asteam turbine and a heat recovery steam generator interconnected in aheat cycle to provide motive energy for driving said turbines, a methodof controlling the load operation of said turbines comprising:a.controlling the temperature of the steam from said steam generator as apredetermined function of the operation of said gas turbine; b.controlling the load rate of said steam turbine as a predeterminedfunction of throttle steam pressure; c. selecting one of a plurality ofcontrol modes and carrying out said temperature and load ratecontrolling steps at different selective levels of automation.
 12. Themethod as described in claim 11, comprising periodically monitoring saidthrottle steam pressure, and periodically carrying out said monitoring,steam temperature controlling, and load rate controlling steps in apredetermined sequence.
 13. The method as described in claim 12, whereinsaid power plant includes an afterburner for receiving exhaust gas fromsaid gas turbine, imparting additional heat thereto and delivering saidheated gas to said steam generator, and comprising the further step ofautomatically coordinating the load operation of said steam turbine, gasturbine and afterburner.
 14. The system as described in claim 1, whereinsaid steam turbine control comprises first means for controlling saidsteam turbine either so that it is held at minimum load or can followsaid gas turbine operation up to maximum load, and second means forcontrolling the load and unload rates of said steam turbine as afunction of sensed steam pressure.
 15. The system as described in claim14, wherein said control system comprises a programmed digital computer,said computer comprising a first subsystem at a first operating levelincorporating said first means, and a second subsystem at a secondoperating level incorporating said second means.